The present invention relates generally to optical disc data storage systems. More specifically, the present invention relates to optical storage systems which utilize a Solid Immersion Lens (SIL) for focusing light onto a data surface of the disc.
Optical data storage disc systems are a promising technology for storing large quantities of data. The data is accessed by focusing a laser beam onto a data surface of the disc and analyzing light reflected from or transmitted through the data surface.
In general, in optical storage systems, data is in the form of marks carried on the surface of the disc which are detected using the reflected laser light. There are a number of different optical disc technologies which are known in the industry. For example, compact discs are currently used to store digital data such as computer programs or digitized music. Typically, compact discs are permanently recorded during manufacture. Another type of optical system is a write-once read-many (WORM) system in which a user may permanently write information onto a blank disc. Other types of systems are erasable, such as phase change and magneto-optic (M-O) systems. Phase change systems detect data by sensing a change in reflectivity. M-O systems read data by measuring the rotation of the incident light polarization due to the storage medium.
The above systems require a beam of light to be focused onto a data surface of a disc and recovering the reflected light. Storage density is determined not only be the size of the markings on the data surface, but also by the size of the beam focused on the surface (i.e. resolution). One type of optical element which can be used in conjunction with an objective lens to reduce the ultimate spot size of the light beam is a Solid Immersion Lens or SIL. A SIL reduces the beam spot size by virtue of the wavelength reduction which occurs when light is inside an optically dense medium. The SIL is positioned very close to the data surface of the disc and couples light to the disc surface via evanescent wave effects. This is often referred to as the "near-field" regime. The use of SILs for data storage is described in U.S. Pat. No. 5,125,750 to Corle et al. which issued Jun. 30, 1992 and in U.S. Pat. No. 5,497,359 to Mamin et al. which issued Mar. 5, 1996. In these optical systems, a laser beam is focused onto the SIL using an objective lens. The SIL is preferably carried on a slider and the slider is positioned close to the disc data surface.
In optical storage systems, it is typically preferred to move the beam between adjacent tracks without moving the slider. This allows more accurate and faster tracking control. However, the SILs of prior art optical systems have been designed to optimize spot size, without regard to off-axis performance.